1. Field of the Invention
The present invention relates to an image processing apparatus of a VTR of a camera integrated type or the like.
2. Related Background Art
FIG. 1 is a block diagram showing a construction of a conventional image pickup apparatus.
In the circuit construction shown in FIG. 1, reference numeral 1 denotes an image pickup element such as a CCD or the like for photoelectrically converting an image and outputting an image signal; 2 a circuit constructed by a double correlation sampling circuit for reducing noises included in the image signal outputted from the image pickup element 1 and an AGC circuit for adjusting a gain of the image signal (hereinbelow, the circuit 2 is referred to as an AGC circuit); 4 an A/D converter for converting the image signal outputted from a clamp circuit 3, which will be explained hereinlater, to a digital signal of eight or ten bits; 5 a signal processing circuit for performing a digital signal process to a digital image signal outputted from the A/D converter 4; 7 a clamp voltage generation circuit for integrating the image signal corresponding to an optical black portion from the image pickup element 1, namely, an optical black level (hereinbelow, referred to as an OB level) of a Y signal every horizontal line and for generating a clamp control voltage on the basis of the integration value; 6 a clamp pulse generation circuit for generating a timing pulse when clamping by a voltage generated by the clamp voltage generation circuit 7; and 3 the clamp circuit for setting a level of the image signal outputted from the AGC circuit 2 to the voltage generated by the clamp voltage generation circuit 7 by the timing pulse generated from the clamp pulse generation circuit 6 (hereinbelow, such a level setting is referred to as clamping).
The specific operation of the conventional image pickup apparatus with such a construction will now be described by also referring to a timing chart of the clamp pulse in FIG. 2.
In the circuit construction of FIG. 1, the image signal outputted from the image pickup element 1 is inputted to the AGC circuit 2 and is amplified to a predetermined signal level. The image signal outputted from the AGC circuit 2 is coupled by a capacitor 13 in an AC manner and is inputted to the clamp circuit 3. The image signal inputted to the clamp circuit 3 is clamped to the clamp voltage generated and outputted by the clamp voltage generation circuit 7 by the clamp pulse (refer to FIG. 2) outputted from the clamp pulse generation circuit 6 in the clamp circuit 3.
The clamped image signal is inputted to the A/D converter 4 and is converted to the digital signal. After that, signal is subjected to a digital signal process by the signal processing circuit 5. The processed signal is recorded by a D-VTR 14 and is also displayed on a monitor 15. The digital signal converted by the A/D converter 4 is also inputted to the clamp voltage generation circuit 7. In the clamp voltage generation circuit 7, the optical black level (OB level) of the Y signal corresponding to an optical black (hereinbelow, referred to as an OB) of the image pickup element 1 is integrated every horizontal line or at preset intervals, thereby generating a clamp voltage such that the integration value is equal to a predetermined value.
The clamp voltage which is generated from the clamp voltage generation circuit 7 constructs a closed loop among the clamp circuit 3, A/D converter 4, and clamp voltage generation circuit 7, so that a feedback is performed so as to hold the OB level of the signal which is inputted to the signal processing circuit 5 to be constant.
The conventional image pickup apparatus performs the clamping of the image signal by the above-mentioned construction.
In the conventional image pickup apparatus, however, as shown in explanatory diagrams with respect to a DC offset amount between the DC voltage and the OB level in a black period shown in FIGS. 3A and 3B, a DC offset exists between the DC voltage (1) in the black period and the OB level (2) of the image signal. Since the DC offset amount (3) [(3)=(2)-(1)] changes depending on the gain of the AGC, a tracing operation of the feedback is disturbed by a time constant when obtaining the OB integration value, so that the clamping operation is not correctly executed.
There is, consequently, a problem such that the OB level of the image signal fluctuates when the signal process is performed, so that the stable image signal cannot be obtained.